By means of tertiary phosphines

Alkylation of tertiary phosphines to quaternary phosphonium salts by alkyl halides is usually a smooth reaction that gives high to quantitative yields.222 For unsubstituted and higher alkyl halides (up to C3) it is best not to use a solvent. 

Secondary and tertiary alkyl halides give olefins on occasions,223 and / -un-saturated alkyl halides give products of allylic rearrangement.224 x,G -Dibromo-alkanes give bisphosphonium salts in satisfactory to high yield.225 Unlike di-bromomethane, tribromomethane reacts with triphenylphosphine only to the monoquaternary salt stage, the reaction being radical jin nature.226 

Preparation of quaternary phosphonium salts is of great importance as it leads to the phosphorus ylides that are needed for Wittig olefination (see page 996). Further, quaternary phosphonium salts are intermediates in the methods of preparation of unsymmetrical phosphines that depend on successive alkylation of tertiary phosphines and degradation of the resulting quaternary salts. Three processes have been developed for the last-mentioned reaction  

Because of the ease of its removal the hydroxymethyl group can even be used for synthesis of phospha analogs of cyanine dyes 2-chloro-benzothia-zolium or -quinolinium salts are treated with 0.5 equivalent of tris(hydroxy-methyl)phosphine in the presence of 1.5 equivalents of a tertiary amine 228 

Arylation of tertiary phosphines cannot be achieved directly by means of aryl halides, but it can be effected photochemically232 or under irradiation233 or above 200° in the presence of aluminum chloride234 or other anhydrous metal halides. 

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